1. Field of the Invention
This invention pertains to document conveying methods and apparatus, and particularly to document conveying methods and apparatus wherein a plurality of superposed strips of documents are conveyed in a direction of conveyance toward a downstream workstation.
2. Prior Art and Other Considerations
U.S. Pat. No. 4,502,676, filed 31 Aug. 1981 by Stocker and incorporated herein by reference discloses a method of operating a document handling machine wherein two strips of documents are conveyed in a direction of conveyance. The two strips are conveyed one over the other in a direction of superposition which is essentially perpendicular to the direction of conveyance. Each of the strips comprises a plurality of documents, each of the documents having respective portions thereon which, upon eventual separation of the document from its strip, will be a leading edge and a trailing edge. Each of the documents belong to one of a plurality of groups of documents with documents in each group being related by a common intended packaging. The strips are conveyed one over the other in the direction of superposition whereby the trailing edges of documents of one strip alternate in sequence in the direction of conveyance with the trailing edges of documents in another strip. The strips are formatted and conveyed whereby during conveyance the trailing edges of all documents in a preceding group are encountered in the direction of conveyance before the trailing edges of documents in a following group are encountered. The term "shingling" is used herein to refer to the just-described method of conveying two superposed strips of documents.
In document handling machines which convey essentially uniformly sized documents in the manner described above it is desirable not only that the trailing edges of documents of a first strip alternate in sequence in the direction of conveyance with the trailing edges of documents in a second strip, but that the two strips be conveyed at essentially the same speed so as to maintain an essentially constant spacing along the direction of conveyance between the trailing edges of documents in the first strip and the trailing edges of documents in the second strip. Preferably the two strips are conveyed at essentially the same speed and in a manner whereby the trailing edges of documents in the second strip are essentially centered between the leading and trailing edges of a document in the first strip; i.e. the trailing edges of documents in the second strip occur essentially half way between the leading and trailing edges of a document in the first strip.
In the past it has been exceedingly difficult to insure that the two superposed strips of documents are conveyed in the direction of conveyance at essentially the same speed. For example, in embodiments wherein two superimposed trips are driven between two rollers comprising the drive mechanism, differing frictional forces affect the speeds of conveyance of the two strips. In such embodiments, a first coefficient of friction occurs between a first such roller and the first strip; a second coefficient of friction occurs between a second such roller and the second strip; and, a third coefficient of friction occurs between the superimposed (i.e. contacting) first and second strips. These three potentially different coefficients of friction tend to cause slippage between the strips, which alters the spacing between the corresponding edges of documents in the two strips.
If the desired constant spacing between the corresponding edges of documents in the two strips as described above is not maintained, then one strip may have a tendancy to "creep" up on the other. The creep may eventually occur to the extent that the corresponding edges of documents in the two strips are essentially aligned in the direction of superposition. Such alignment of leading edges causes problems when a downstream operation such as bursting, for example, is being performed in alternating strip fashion with respect to the documents in the two superposed strips. Alignment of the leading edges of documents in the two superposed strips can disrupt the bursting process and can confuse downstream document handling machinery (such as folders and collectors) regarding the proper sequence of documents to be handled thereby.
Heretofore one method of preventing alignment of the leading edges of documents in the two superposed strips was the essentially constant visual surveillance of the two conveyed strips. If the leading edges of documents in one strip began to creep up on the leading edges of documents in the other strip, the document handling machine was shut down so that the two strips could be repositioned relative to one another. The repositioning of the two strips was only a temporary preventative, however, in that the cause of the creep--the differing speeds of conveyance of the two strips--was not rectified.
In the above regard, in apparatus wherein the two strips are conveyed by power-driven roller-type mechanisms, causes of the differing speeds of conveyance of the two strips include the uneven wear of one or more rollers and the build up of dirt or the like on the rollers. Hence, in the prior art, efforts were made to keep the rollers clean and free from debris or the like that would influence the speed of strips conveyed by the rollers. These efforts were, for the most part, tedious and imprecise.
In view of the foregoing, it is an object of this invention to provide a document conveying method and apparatus wherein the conveyance of superposed strips of documents is controlled in a manner whereby the leading edges of documents in one strip are not prone to creep up on the leading edges of documents in the other strip.
An advantage of the present invention is the provision of a document conveying method and apparatus which automatically controls the relative speeds of conveyance of superposed strips of documents.
Another advantage of the present invention is the provision of a document conveying method and apparatus which effectively monitors the relative speeds of conveyance of superposed strips of documents.
Yet another advantage of the present invention is the provision of a document conveying method and apparatus wherein conveying speeds of superposed strips of documents are easily established upon apparatus start up.
Another advantage of the present invention is the provision of a document conveying method and apparatus which combats uneven wear of roller members comprising the apparatus.
Still another advantage of the present inventon is the provision of a document conveying method and apparatus which requires little operator supervision and surveillance.